Balling phenomena is a common defect in selective laser melting (SLM) process, it is the main reason causing high surface roughness、pores、tilt delamination and other defects, seriously affect the density and mechanical properties of parts. The classification and formation process of balls、influencing factors and hazards of balling phenomena at home and abroad are summarized systematacially. Results show that different metal balls have different formation mechanisms. The formation process of balling phenomena is complex, and many factors during fabricating affect the formation of ball reaction. But the current researches at home and abroad of balling phenomena are not systematic, comprehensive and thorough enough. It will be useful for restraining and eliminating balling phenomena during SLM process to research on the influence of technological parameters, physical and chemical features of powders on balling.